Hybrid Striped Bass Morone chrysops x M. saxatilis are popular sport- and food-fish commonly reared in the U.S., and have been exported for culture in the East (particularly Taiwan), Middleast, and Europe. Hybrid Striped Bass are typically reared on diets containing considerable amounts of both fish meal and fish oil. Fish meal sparing research has suggested that Hybrid Striped Bass accept a range of alternative, terrestrial-origin protein sources, including grain distillers dried yeast (Gause et al. 2011a, 2011b). However, in these cases, fish oil was commonly used as the primary dietary lipid source.

At higher inclusion rates (~36-50%), grain distillers dried yeast-based feeds yield reduced performance in Hybrid Striped Bass, but at more moderate inclusion levels (~13-27%) like that used in the present work (15%), grain distillers dried yeast appeared to enhance feed consumption and growth (Gause and Trushenski 2011a, 2011b; Trushenski and Gause 2013).

Grain distillers dried yeast has also proven similarly effective when incorporated at moderate rates in feeds for Rainbow Trout (Hauptman et al. 2014) and Pacific White Shrimp (Achupallas 2013). Thus, the improved performance we associated with partially sparing fish meal with grain distillers dried yeast in the present work is consistent with previous testing of this ingredient in aquafeeds.

It is well-documented that the tissue fatty acid profiles of fish are affected by dietary fatty acid intake, and the influence of diet on tissue composition of Hybrid Striped Bass has been demonstrated many times using various dietary lipid sources. However, dietary fatty acid composition does not directly or proportionately influence tissue composition in all cases.

Standard soybean oil contains no LC-PUFAs, but is rich in 18:2(n-6); and fillets of fish fed diets containing standard soybean oil became enriched with 18:2(n-6) at the expense of LC-PUFAs.

Vegetable oil refinery lipid, being primarily soy-based, yielded similar results. Hydrogenated soybean oil doesn’t contain LC-PUFAs either, but instead contains high levels of 18:0; however, 18:0 did not become significantly enriched in fillets of fish fed these diets and only a marginal reduction in fillet LC-PUFAs was observed.

Moreover, the degree of tissue profile distortion, as indicated by Djh values, was markedly lower among fish fed the hydrogenated soybean oil-based feeds. Similar results have been reported in previous investigations of hydrogenated soybean oil and other SFA-rich lipids in aquafeeds for Hybrid Striped Bass and other taxa (Trushenski et al. 2008; Trushenski and Kanczuzewski 2013; Kanczuzewski and Trushenski 2015; Trushenski et al. 2015). Given the apparent strategic advantage of hydrogenated soybean oil over other alternative lipids, there is growing interest in using this and similar feedstuffs in aquafeeds. We have advocated caution in incorporating these ingredients at high inclusion rates, given that digestibility may be a limiting factor for SFA-rich lipids (Kanczuzewski and Trushenski 2015). We believe some degree of caution is still warranted, however, the absence of negative, lipid-related effects on growth in the present case is encouraging.

We previously investigated sparing of marine feedstuffs with poultry byproduct meal and poultry fat in Hybrid Striped Bass (Trushenski and Kohler 2011) using a factorial approach similar to that used in the present work. We previously concluded that the effects of dietary lipid source and dietary protein source on fillet fatty acid composition were, in essence, the same: both were the result of shifts in dietary fatty acid composition. When fish oil is replaced with an alternative lipid source such as poultry fat, the fatty acid composition of the feed is altered. Although the effect is less overt, replacing fish meal with an alternative protein source such as poultry byproduct meal also affects the fatty acid composition of the feed via the small amounts of residual lipid that are present in both protein meals.

The present work illustrates the same principle: switching dietary lipid sources (i.e., fish oil and the soybean-derived lipids) substantially alters the fatty acid composition of the feed and, in turn, the fillets (with the exception of the hydrogenated soybean oil-based feeds noted above); switching other ingredients which contain some lipid (i.e., fish meal and grain distillers dried yeast) will have a similar, though less dramatic effect on dietary and tissue composition.

As noted by Trushenski and Kohler (2011), the effects of fish oil sparing and fish meal sparing on tissue composition in Hybrid Striped Bass are statistically independent, but also causally linked by the fatty acids contained within these ingredients and their alternatives.

In conclusion, our experiment provides further evidence in support of Hybrid Striped Bass’s acceptance of a range of dietary protein and lipid sources, the largely independent nature of fish meal and fish oil sparing on growth performance and tissue composition of fish, and the utility of alternative ingredients such as grain distillers dried yeast and various soy-based lipids in feeds for Hybrid Striped Bass.